An electrohydraulic servovalve is basically a device for converting an electrical input signal into a proportional hydraulic output.
In a two-stage servovalve, a first- or pilot-stage is used as a hydraulic amplifier to selectively displace a second- or output-stage valve spool relative to a body. The pilot-stage typically has a torque motor operatively arranged to vary the position of a member in proportional response to a supplied electrical current. This member may be a flapper positioned between a pair of opposed nozzles, a deflector-jet, a jet pipe, or the like. In any event, the pilot-stage is employed to produce a pressure differential proportional to the supplied current, which differential is then applied to the end faces of the second-stage valve spool and used to selectively displace the spool relative to the body. Thus, the pilot-stage pressure differential creates a force which urges the valve spool to move from an equilibrium position, while the flow into the expanding spool end chamber and from the contracting spool end chamber determines the velocity of the spool relative to the body. The spool is, in turn, operatively arranged to control the hydraulic output (i.e., pressure, flow, etc.) of the servovalve.
Such servovalves are also classified by type. See, e.g., Technical Bulletin 103, "Transfer Functions for Moog Servovalves", Moog Inc. (1965). For example, a "flow-control" servovalve is one in which flow through the second-stage at constant load is substantially proportional to the supplied electrical current. These valves typically have a spool position feedback loop closed about the spool and torque motor such that the magnitude of spool displacement off null will be substantially proportional to the magnitude of the supplied current. The direction of spool displacement is determined by the polarity of the supplied current. The feedback mechanism may be in the form of a mechanical spring wire connecting the spool with the flapper (See, e.g., U.S. Pat. No. 3,023,782), or in the form of centering springs operatively positioned in the spool end chambers, or may be electrical (see, e.g., U.S. Pat. No. 3,752,189). Thus, in such a "flow-control" servovalve, the magnitude of spool displacement off null is substantially proportional to the magnitude of the supplied current. The effect of such spool displacement is to create ports or orifices through which fluid may flow. The area of such ports or orifices is generally proportional to the magnitude of spool displacement. However, flow through the second-stage is a function of such orifice area and the pressure differential there-across. Hence, if the load is constant, the pressure differential across the second-stage ports will normally be constant, and flow will be proportional to spool displacement and input current. On the other hand, if such pressure differential varies (e.g., as by the load varying), then flow through the second-stage will also vary. Nevertheless, this type of valve is commonly referred to as a "flow control" valve because, at constant load (i.e., at constant pressure differential) second-stage flow is substantially proportional to the supplied current.
In a "pressure-control" servovalve, the hydraulic output of the second-stage is a differential pressure provided to the load. Thus, a four-way "pressure-control" servovalve will produce a differential pressure output proportional to the supplied current. This type of valve is structurally similar to a "flow-control" valve, except that the spool position feedback loop is omitted, and a pressure feedback loop is closed about the second-stage spool and the load (see, e.g., U.S. Pat. No. 2,931,389). In effect, the current supplied to the torque motor causes a first axial force to be applied to the spool. However, the load pressure differential exerts an opposing second force on the spool. These two forces equal one another when the load pressure differential equals the torque motor-produced differential. However, there is typically no feedback connection between the spool and the torque motor, such as a spring wire or centering springs in the case of a "flow-control" servovalve, and the second-stage spool will move to an equilibrium position relative to the body at which such opposing fluid forces balance one another.
A "pressure-flow" (PQ) control servovalve is somewhat of a hybrid design, which combines the functions of pressure and flow control and contributes effective damping in highly-resonant loaded servosystems. Flow from these servovalves is determined not only by the magnitude of the supplied electrical current, but also by the differential load pressure. Such PQ servovalves have been provided with a pressure feedback loop (i.e., as in the case of a "pressure-control" servovalve), and with a mechanical spool position feedback loop (i.e., as in the case of a "flow-control" servovalve). Such PQ servovalves have heretofore been deliberately designed to have pressure-flow characteristics intermediate those of a conventional "flow-control" and "pressure-control" servo-valves. As will be demonstrated infra, a "flow-control" servovalve has a high pressure gain at low currents. While this is desirable in some applications (e.g., to overcome threshold or static friction), it can produce problems in others. For example, if two "flow control" servovalves are used in parallel to control the position of a common load, a differential in the pressure-to-current gains of the valves can produce force-fighting between the valves. As indicated above, such pressure gain varies inversely with current in conventional "flow-control" valves. In other words, for small currents, the pressure gains are high and the problem of force-fighting between the valves is amplified. Because of this, the PQ valve was developed largely to reduce the pressure gain at small currents and to provide a valve suitable for use in such redundant or tandem applications.
In some actuator systems, it is sometimes desired to provide for redundant operation. This is common in aircraft applications, particularly with the advent of fly-by-wire control systems. Various types of redundant control systems are shown and described in Technical Bulletin 105, "New Servovalves for Redundant Electrohydraulic Control", Moog Inc. (1963). In such redundant control systems, it is generally desired to equalize the operation of multiple servovalves so as to avoid force-fighting therebetween at the actuator.